This invention relates to semiconductor device contacts and more specifically relates to a novel expansion contact for a high power semiconductor device die.
High power silicon die such as diode die, power MOSFET die, IGBT die and thyristor die are operated at relatively high temperatures and experience considerable thermal expansion and contraction. Electrical contacts to the die surfaces must provide a low ohmic resistance contact and should not exert high differential expansion and contraction stress on the die since such stress can damage or crack the fragile die.
Power MOSFETs and IGBTs are often provided with ultrasonically formed aluminum wire bonds to their aluminized source or emitter surfaces. This is a relatively small area bond which will not overly stress the underlying silicon. However, when lower contact resistance is needed, large area copper electrodes, for example, are soldered, or connected to the die top electrode surface by a conductive epoxy. The solders used are xe2x80x9csoftxe2x80x9d solders so that the solder, like the epoxy, will absorb a certain amount of the differential stress caused by the differential expansion of the copper and of the silicon die.
It is known to use materials having thermal expansion characteristics like that of silicon, such as molybdenum, tungsten or nickel-iron alloys as an intermediary between a silicon die contact surface and a larger contact body. These arrangements are principally used in the thyristor art and are very expensive.
It would be very desirable to provide a contact structure for connection to selected areas of a semiconductor die which will not cause differential thermal expansion and contraction stress and which is inexpensive and is adaptable to mass production techniques.
A novel contact (sometimes termed a xe2x80x9cclipxe2x80x9d) is provided for the active surface area of a silicon die such as a power MOSFET which consists of a xe2x80x9csandwichxe2x80x9d of a low expansion material, particularly a nickel-iron alloy, which is coated on its opposite surfaces by a highly electrically conductive layer of metal. In the preferred embodiment of the invention a nickel-iron alloy known by the trade name xe2x80x9cNiLo alloy 42xe2x80x9d is coated with a conductive layer of silver, gold or copper. This conductive layer is thin and soft relative to the thickness and hardness respectively of the nickel-iron body. Thus, the thermal expansion properties of the clip will be nearer to those of the nickel-iron alloy while its electrically conductive properties will be closer to those of the coating. By way of example, the nickel-iron alloy thickness may be close to that of the die, and may be about 15 mils. The coating thickness, however, can be about 2 mils or less. The clip can be bent to cover only the desired active area of the die to which it is connected.